This invention relates to the art of quenching, that is, the removal of heat from an article, usually at a rapid rate, in order to alter its physical properties or simply to cool it. Although not limited to such, the glass and metals industries are the major users of quenching processes. With glass, quenching is used in the tempering process to rapidly cool a heated piece of glass so as to cause the formation of a surface layer stressed in compression which strengthens the glass. Metals are quenched to rearrange their crystal structures and thereby harden the metals. Another application of quenching is for the purpose of quickly solidifying thermoplastic articles immediately after shaping so as to permit handling thereof. An example of this latter application is the catching of falling droplets of molten material such as glass, metal, plastic, or bituminous materials.
Quenching has been carried out in the prior art with a wide variety of fluids, most commonly air, water, or oils. Examples of more exotic quenching media are sublimable particles (e.g., frozen CO.sub.2) as disclosed in U.S. Pat. No. 3,764,403 to Neely and streams of plastic particles as disclosed in U.S. Pat. No. 3,423,198 to McMaster. For most quenching purposes, it is desirable for the quenching fluid to provide a maximized rate of heat transfer from the article being quenched, but it is also usually important for the cooling to take place as uniformly as possible to avoid the creation of unbalanced stresses in the article which may weaken or even break the article. Water is capable of producing very rapid cooling rates, but surface boiling phenomena which accompany water quenching cause abrupt, localized changes in the heat transfer rate which lead to imbalanced stress conditions in the article and sometimes breakage. Therefore, water is not a suitable quenching medium for many materials which cannot withstand such stresses. The use of viscous liquids such as oils have been resorted to in order to obtain more uniform heat transfer, but with some reduction in the rate of heat transfer. However, perhaps the greatest drawback to oils and the like as quenching media is the need to subsequently wash the articles to remove the quenching liquid from their surfaces. Air has a relatively low heat transfer coefficient and therefore usually requires high velocities to be useful as a quenching medium. Such high velocity streams of air are difficult to apply to an article uniformly, and even with the highest practical air velocites, the heat transfer rates obtained are seldom as high as those obtained with liquids and considerably less than with water. Moreover, the buffeting caused by high velocity streams of air can sometimes cause distortion in heat-softened glass.
Numerous proposals have been made to utilize particulate materials entrained in or fluidized by air as heat transfer media. The particulate materials proposed have included finely divided alumina or silica, fine sand, metallic powders, powdered graphite, and plastic particles as disclosed in U.S. Pat. No. 3,423,198. While such materials have been able to improve the heat transfer rate of air alone, they generally do not provide rates as high as liquids. Additionally, the impact of solid aggregates against the surfaces of the articles being quenched can produce defects in some articles, especially glass. Also, adhesion of powdered materials to the articles being quenched can sometimes be a problem. This adhesiveness has been deliberately employed to coat falling droplets with fluidized beds of coating materials as shown in U.S. Pat. Nos. 3,070,837; 3,202,731; and 3,791,987.
U.S. Pat. No. 3,764,403 discloses a quenching method which utilizes solid particles while avoiding distortion-causing impacts. This is accomplished by using particles which are sublimable solids, such as frozen carbon dioxide, which behave like solids in improving heat transfer through the gas boundary layer at the surface of the article being quenched, but which instantly sublime to the gaseous state as they approach the hot surface of the article itself, thereby muting any impact effect. Such an approach is limited in its acceptability by the high cost of the sublimable particles which are expended and escape as gases. It would be highly desirable if such a soft-impact particulate quenching medium capable of being recirculated were available.
The quenching medium of the present invention is a fluidized dispersion of water in hydrophobic colloidal silica. Hydrophopic colloidal silica is a commercially available product used in a wide variety of applications such as anti-caking agent in granular materials, water-repellancy additive in paints and sealants, and reinforcing agent in polymers. The formation of a dispersion of water in hydrophobic colloidal silica has been disclosed in U.S. Pat. No. 3,393,155, and the utility for such dispersions are indicated therein to be as fire extinguishing agents, for treating papers and textiles, and for use in rubbers or plastics or polymer dispersions. It has also been disclosed in U.S. Pat. No. 3,710,510 that dispersions of water in hydrophobic colloidal silica may be used to germinate seeds. These prior art dispersions inherently include substantial amounts of air and are in a fluid-like state, but not in a state of dynamic fluidization as used in the present invention. Furthermore, the prior art did not recognize the potential as a quenching medium for such dispersions when fluidized.